Recruitment failure and shifts in community structure following mass mortality limit recovery prospects of black abalone

Mass mortalities of species can fundamentally alter the structure of natural communities, which can in turn negatively impact species' recovery. Beginning in 1994, some of the largest remaining populations of black abalone Haliotis cracherodii on the mainland coast of California, experienced mass mortalities due to the fatal disease called 'withering syndrome', which led to its listing as a species of concern by the USA National Marine Fisheries Service. We have been monitoring black abalone populations along the coast of southern and central California since 1992, and detection of withering syndrome at our southernmost site prompted us to investigate how the impending decline of this dominant grazer might correlate with changes in black abalone recruitment and the rocky intertidal community in which it lives. Quantitative surveys before and after mass mortalities revealed that, after black abalone declined, there was a shift in the composition of the intertidal species assemblage from one dominated by bare rock and crustose coralline algae (good quality abalone habitat) to one with increased cover of sessile invertebrates and sea urchins. Declines in abalone abundance were also correlated with a lack of recruitment to areas affected by withering syndrome, despite the presence of healthy adult populations only tens of kilometers away. This suggests that abalone recruitment might be limited by dispersal, a lack of quality habitat for settlement and early survival, or the continued presence of the disease agent. Recruitment failure and these dramatic shifts in habitat quality indicate that the outlook for recovery of black abalone is poor.